<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        div{
            width: 100px;
            height: 100px;
        }
        .hide{
            background-color: green;
        }
        .show{
            background-color: red;
        }
    </style>
</head>
<body>
    <div class="erweima hide" id="er"></div>
    <script>
        var er = document.getElementById("er");
        er.className = "erweima show"
    </script>
</body>
</html>